First day potty training - is this normal?

I tried to start potty training my 2.5 year-old son today. He was very excited about using the potty yesterday when I was talking to him about it, and we have been prepping him for a couple weeks by reading books and watching videos about the process.

This morning I put them in underwear and told him we would be using the potty today. Every 15 to 30 minutes I would take him to try to go. He did have three accidents in his underwear (which I expected). But each time it happened less than two minutes after I tried to have him go on the potty. He did get half a poop in the toilet because I caught him trying to go in his underwear and ran him there.

After nap, I decided to try no underwear (naked from waist down) because I thought they might remind him too much of a diaper. He did not pee the entire time after nap until bed. I feel like he was withholding it. We continued trying to go in the potty, but had no luck. He was getting frustrated. But he didn’t even have an accident at all.

So at the end of the day, he didn’t pee in the toilet once and showed no signs of almost going. I couldn’t even show him that pee is supposed to go in by catching him mid pee. Is this normal for the first day? I’m going to try to continue for the next couple days, but maybe he just isn’t ready? Any thoughts or tips?